1. Field of the Invention
The present invention relates to organolithium initiated anionic polymerizations of monomers in a hydrocarbon solvent terminated in the presence of metal alkyls. More particularly, the present invention relates to a method of synthesizing high molecular weight end-functional polymers by organolithium initiated anionic polymerizations terminated by carboxyl functional terminators.
2. Technical Background
Anionic polymerization of monomers in the presence of an anionic polymerization initiator is a widely used commercial process. When solution anionic techniques are used, polymers are prepared by contacting the anionically polymerizable monomers simultaneously or sequentially with an anionic polymerization initiator. Particularly effective anionic polymerization initiators are organolithium compounds having the general formula: RLin, wherein R is an aliphatic, cycloaliphatic, aromatic or alkyl-substituted aromatic hydrocarbon radical having from 1 to about 20 carbon atoms and n is an integer of 1 to 6.
Such lithium initiated anionic polymerization processes are frequently complicated by side reactions during the major portions of the process, including: side reactions during the growth of the lithium polymers; side reactions during the termination process; and side reactions after termination. Some of these side reactions cause coupling of living polymer chains. This is an impediment to the goal of making macrostructures with well defined molecular weights and high concentrations of functional end groups. Side reactions of interest can be classified as either readily reversible reactions, such as aggregations, or as generally irreversible reactions such as coupling or cross-linking. Prior art has identified and addressed some of these undesirable side reactions.
For example, prior art teaches anionically polymerizing at least one monomer in the presence of an organolithium initiator in a hydrocarbon solvent so as to produce a living polymer. The living polymer C—Li ends are then capped with a capping agent or with a terminating agent that produces a functional end group. Where a multifunctional organolithium initiator is used, strongly-associating chain ends are formed such that the polymer and solvent form a polymer gel.
It is known to prevent the formation of such polymer gel by the addition of a trialkyl aluminum compound or a similar alkali metal compound during the polymerization/capping process. In one process, a trialkyl aluminum compound is added to the polymer gel such that it dissipates the gel. In another process, the aluminum trialkyl may also be added before or during polymerization or before or with the capping agent.
In an example of another undesirable side reaction, the presence of C-alkali metal chain ends, and for example C—Li chain ends, in the living polymer during the polymerization contributes to the aggregation of polymer-alkali metal centers in a reversible side reaction that increases the viscosity of certain anionic polymerization solutions during the polymerization process. Where organolithium initiators are used, C—Li chain ends are the points in the molecule of the initiator where the carbon-lithium bond is located and at which the propagation of the polymer chain occurs. Certain lithium alkyls form aggregates in hydrocarbon solution. These aggregates are stabilized by metal-metal bonding between the lithium centers in the lithium alkyl moieties. When such aggregates are present in living anionic polymer solutions, the equilibrium between aggregated and unassociated polymer chains appears to strongly favor the aggregated species. The unassociated species, though present as a minor component of the mixture, is the only center for propagation of polymerization.
In solutions of living lithium polymers the aggregation of polymer-lithium centers can be described in the reversible side reaction:(Polymer-Li)n⇄n Polymer-Li.The aggregated moiety is n times larger in mass than the unassociated polymer, n representing the number of centers in the aggregate. For this reason, the viscosity of living anionic polymer cement (mostly aggregates) is typically higher than that of its terminated analog (as a terminated chain would have no C—Li centers, it has been generally assumed that a terminated chain will not be aggregated).
It is also known to reduce the aggregation of polymer-lithium centers by the addition of selected metal alkyls to solutions of living anionic polymers. The metal alkyl is selected from the group of metal alkyls that interact with C—Li centers to form metal “ate” complexes. The metal alkyl compounds are selected to be less basic and/or less bulky or both than the organo substituents of the alkali metal compound. The organo substituents of the alkali metal compound are aliphatic, cycloaliphatic, aromatic, or alkyl-substituted aromatic.
In one example of this type of interaction, an aluminum alkyl is used as an example of the preferred type of metal alkyl and a living anionic polymer chain end as the preferred type of aggregation prone lithium species. In solutions of living lithium polymers the formation of aluminate complexes can be described in the reversible side reaction:Polymer-Li+R3Al⇄Polymer-AlR3Li.The “ate complex” is in equilibrium with the unassociated polymer chain. It is important that this complex is formed reversibly as the “ate complex” is not capable of either initiating or propagating the polymerization of monomer.
The equilibrium concentration of aggregated polymer-lithium species is reduced in preference to equilibrium formation of the aluminate complex. As more aluminum alkyl is added, more aggregated polymer-lithium species are eliminated, up to the limit where all of the polymer-lithium is associated with the aluminum center. This occurs at about one aluminum alkyl per polymer lithium center. Minimum effective amounts of metal alkyl are in the range of 0.01 equivalents of metal alkyl per 100 alkali metal centers. Generally, it is preferred that no more than 1.5 equivalents of metal alkyl be used, most preferably, no more than 1 equivalent. Whether the metal alkyl is added before or at the beginning of polymerization or during or after polymerization, the basis is equivalents of alkali metal.